Arteriovenous malformations (AVMs) are a developmental anomaly of the vascular system, consisting of tangles of poorly formed blood vessels in which the feeding arteries are directly connected to a venous drainage network without any interposed capillary system.
AVMs can occur anywhere in the body, however, brain AVMs are of special concern because of the inherent high risk of bleeding of the abnormal blood vessels that can cause neurological damage.
Not much is known about the etiology of brain AVMs. The cause of brain AVMs is yet unknown, however, it is possibly multifactorial; apparently both genetic mutation and angiogenic stimulation (the physiological process of formation of new blood vessels from pre-existing vessels) playing roles in AVM development. Some believe that AVMs develop in utero. While others advocate an angiopathic reaction, following either a cerebral ischemic or hemorrhagic event (subtypes of stroke) as a primary factor in their development.
The incidence in the United States is 1.34 per 100,000 person-years, although the actual prevalence rate is higher due to clinically silent disease, as only 12% of AVMs are estimated to become symptomatic. The mortality rate is 10-15% of patients who have hemorrhage, and morbidity varies from approximately 30-50%. There is no sex predilection. Despite the considered congenital origin of AVMs, the clinical presentation most commonly occurs in young adults.
Arteriovenous malformations are composed of a central vascular nidus which is a conglomerate of arteries and veins. There is no intervening capillary bed, and the feeding arteries drain directly into the draining veins by one or multiple fistulae. These arteries lack the normal muscularis layer and the draining veins often appear dilated due to the shunted high-velocity arterial blood flow entering through the fistulae.
AVMs cause neurological dysfunction through the following three possible pathophysiological mechanisms. Firstly, the abnormal blood vessels have a propensity to bleed resulting in hemorrhage occurring in the subarachnoid space, the intraventricular space or, more commonly in the brain parenchyma. Secondly, in the absence of hemorrhage, seizures may occur as a consequence of the mass effect of AVM or venous hypertension in draining veins. The third important cause of slowly progressive neurological deficits is attributed to the"steal phenomenon" which is thought to be related to normal brain parenchyma deprivation from nutrients and oxygen, as blood bypasses the normal capillary bed to the malformed arteriovenous channels.
AVMs tend to be clinically asymptomatic in 15% if cases until the presenting event occurs.
Studies report seizure as a presenting disorder in 15-40% of patients. The risk of seizures increases with cortically-located, large, multiple, and superficial-draining AVMs. Seizures are typically focal, either simple or partial complex, but often show secondary generalization.
The progressive neurological deficit may occur in 6-12% of patients over a few months to several years. A vascular steal syndrome has been hypothesized to cause this presentation, but in most cases, this is related to mass effect, hemorrhage, or seizure. These include seizure, hemiparesis, visual disturbances, loss of sensation in one-half of body and aphasia. Minor bleeding can occur with no noticeable symptoms.
Brain AVMs are typically first identified on cross-sectional imaging (CT or MRI). A combination of MRI and angiography are often helpful to plan therapy and predicting the likely success and associated risks of surgical, endovascular, or radiological therapy.
Computed tomography — On non-contrast CT the nidus is blood density and therefore usually somewhat hyperdense compared to adjacent brain, enlarged draining veins and calcification may be evident. Although many of them are large, however, no mass effect or edema is present unless they bleed. On postcontrast CT especially with CT angiogram, the diagnosis is easily derived with visible feeding arteries, nidus and draining veins apparent in the so-called "bag of worm" appearance. The exact anatomy of feeding vessels and draining veins can usually be delineated with angiography. The sensitivity of CT to identify brain AVMs in the acute setting of hemorrhage is reduced owing to compression of the nidus by the hematoma so more sensitive techniques such as MRI or angiography are required.
Magnetic resonance imaging — MRI is very sensitive for plotting the location of the brain AVM nidus and often an associated draining vein or any distant bleeding event. Fast flow in a conglomerate of tangled blood vessels generates serpiginous and tubular flow voids seen on bothT1 and T2, however mostly evident on T2 weighted images. Complications like the previous hemorrhage, adjacent brain edema and atrophy may be seen.
After radiosurgery, MRI can evaluate the regression of the nidus volume, post therapy edema as well as the radiation necrosis in the radiation field.
Angiography — It remains the gold standard for the diagnosis and treatment planning. Nidus configuration, its relationship, and drainage to surrounding vessels are precisely evaluated. The presence of associated aneurysm suggests a higher risk for hemorrhage. Contrast transit time, which relates to the flow state of the lesion, can provide the critical information for the endovascular treatment planning.
Treatment modalities —Invasive management is recommended for younger patients with presence of one or more of the high-risk features for an AVM rupture, whereas in the case of older individuals with no high-risk features, the usual best treatment is medical management. In these particular patients, anticonvulsants for seizure control and pertinent analgesia for headaches may be the only management required. Several studies report that a history of the previous rupture is one of the most significant risk factors that predict the long-term bleeding risk. Others important factors include patient age, AVM location, the presence of aneurysms, size, and other vascular features. Patients with AVMs and intractable epilepsy are also candidates for AVM treatment.
Surgical excision — Open microsurgical excision is the mainstay of treatment and offers the cure for patients considered to be at high risk of hemorrhage.
Spetzler-Martin Grade (SMG) scale is commonly employed for assessment of the risk of surgical morbidity and mortality with brain AVMs. It is a composite score of nidus size (<3 cm, 3-6 cm, >6 cm; 1-3 points), eloquence of adjacent brain (1 point for lesions located in brainstem, cerebellar peduncles, thalamus, hypothalamus, or language, sensorimotor, or primary visual cortex), and venous drainage (1 point if any or all of venous drainage is via deep veins, such as basal veins, internal cerebral veins, or precentral cerebellar veins). The higher the score, the higher the associated surgical morbidity and mortality risk.
Radiotherapy and endovascular embolization are not only useful alternatives to the surgical treatment in patients at high risk for surgical therapy but can also be useful adjuncts to the main surgical management.